Office chairs have been developed where seat and back assemblies thereof are tiltable forwardly and rearwardly. One type of office chair is commonly referred to as a synchro-tilt type chair wherein the back assembly tilts synchronously with respect to the seat assembly but at a greater rate As a result, the back assembly tilts relative to the seat assembly as the latter tilts relative to a chair base on which the seat and back are supported. Such synchronous tilting is provided by a tilt control mechanism which mounts to the chair base and joins the back assembly to the seat assembly. Numerous control mechanisms have been developed which effect such tilting.
More particularly these tilt control mechanisms typically include a spring arrangement contained therein which resists the rearward tilting of the seat and back. Preferably the spring arrangements cooperate with a spring adjustment mechanism so as to adjust the load of the spring which resists the rearward tilting. Thus, the amount of force necessary to tilt the seat rearwardly can be manually adjusted to suit each user.
Typically these spring adjustment mechanisms include handles which project out of the tilt control mechanism housing and are rotatable so as to vary the spring load. While a large number of these adjustment mechanisms use adjustment knobs which project downwardly through the bottom of a control housing, providing the adjustment knobs on the side of the tilt control mechanism is easier to operate since a user need not reach down below the seat
Examples of tilt control mechanisms having side tension adjustment mechanisms are disclosed in U.S. Pat. Nos. 4,865,384, 4,889,384, 5,106,157, 5,192,114 and 5,385,388.
Accordingly, it is an object of the invention to provide an improved tilt control mechanism for an office-type chair which preferably is a synchro-tilt control It is a further object that the tilt control mechanism include a side-actuated tension adjustment mechanism which acts upon a spring arrangement to vary the spring force tending to urge the seat assembly to a normal forward position. To optimize the space required for the tilt control mechanism, it is a further object that the control mechanism have a low-profile design wherein a combination of front and rear springs is provided. In view thereof, it is an object of the invention that the tension adjustment mechanism act on either the forward or rearward springs. A still further object is to provide a tilt control mechanism wherein the spring arrangement urges the seat forwardly but provides for a drop-off or dwell in the spring load being applied once the seat reaches a rearward position such that the seat can be readily maintained in the rearward position with less force than was required to move the seat to the rearward position.
In view of the foregoing, the invention relates to a tilt control mechanism for a chair which provides for synchronous tilting of the seat and back assemblies. Preferably the tilt control mechanism is supported on a chair base while the seat assembly and back assembly are joined together by the tilt control mechanism. The tilt control mechanism disclosed herein permits both rearward tilting of the seat relative to the chair base while also permitting a corresponding rearward tilting of the back assembly relative to the seat. The tilting of the back assembly is at a different and preferably greater rate than the rearward tilting of the seat which is commonly referred to as "synchro-tilt". The tilt control mechanism also permits forward tilting of the seat relative to the base to further optimize the comfort of a user.
More particularly, the tilt control mechanism includes a box-like control housing which is rigidly secured to the base. The control housing opens upwardly to define a hollow interior and contains the internal components of the tilt control mechanism.
To effect rearward tilting, the control mechanism includes a seat back support member which is hinged to the control housing by a center pivot rod, screws or the like. The back support member extends rearwardly therefrom to support the back assembly. In particular, the center pivot rod defines a first horizontal pivot axis so as to permit vertical swinging of the back support member about this horizontal pivot axis. The back support member forms a lower generally horizontal leg of an L-shaped back upright which supports the back assembly thereon. Thus, the back assembly tilts rearwardly in response to a corresponding swinging movement of the back support member.
The control mechanism further includes a horizontally enlarged top plate which has a front edge portion pivotally secured to the control housing by a front pivot rod, and a rear edge portion slidably secured to the back support member by a rear pivot rod, screws or other suitable fasteners In particular, the rear edge portion of the top plate includes horizontally elongate slots which are formed through the side walls thereof and slidably receive the opposite ends of the rear pivot rod therethrough. Unlike the center and front pivot rods which only provide for pivoting movement, the opposite ends of the rear pivot rod project from the back support member and are movable forwardly and rearwardly along the slots formed in the top plate. Preferably, the opposite ends of the rear pivot rod includes bearings or rollers that roll along the slots so as to reduce friction. Thus, while the control housing remains stationary, the top plate and back support member pivot downwardly together but at different rates during rearward tilting of the chair. While this movement is in a downward direction, the rearward tilting of the seat and back occurs Similarly, upward pivoting of the top plate and back support member effects a forward tilting of the seat and back.
To normally maintain the back assembly in an upright position, the control mechanism includes a front coil spring supported on the front pivot rod, and a pair of rear coil springs supported on the rear pivot rod. These coil springs include lower legs which act downwardly on the stationary control housing and upper legs which act upwardly on the pivotable top plate The front and rear coil springs thereby urge the top plate as well as the back support member upwardly relative to the stationary control housing. The springs, however permit rearward tilting of the top plate and the back support member.
The tension being applied by the coil springs is adjusted by a tension adjustment mechanism. The tension adjustment mechanism includes a wedge block which preferably seats underneath the lower legs of the front springs, and a side-actuatable adjustment rod which is movable laterally into and out of the control housing to move the wedge block forwardly. To transform the lateral movement of the rod into the forward movement of the wedge block, the wedge includes an angled groove on a bottom surface thereof which is seated on an elongate track that projects upwardly from the control housing. The track extends at an angle toward the front of the control housing, and the wedge slidably seats on the track such that the wedge block is slidable therealong at an angle relative to the coil springs. Thus, upon sideward movement of the adjustment rod, the wedge block is moved both sidewardly and forwardly as it travels along the angled track wherein the forward movement of the block tends to urge the lower spring legs upwardly and increase the spring force being applied thereby.
To minimize the effects of the sideward movement of the wedge block on the spring legs, an intermediate plate is disposed between an inclined front surface of the wedge block and a lower surface of the spring legs. By providing the intermediate plate, the sideward movement of the wedge block does not tend to urge the spring legs sidewardly as would otherwise occur if the wedge block acted directly on the spring legs. This tension adjustment mechanism thereby permits ready adjustment of the force provided by the front coil springs.
A further aspect of the chair is provided by the rear springs wherein the lower legs of the springs act upon the control housing, and in particular, act upon an arcuate bearing surface that is supported on a rear edge of the control housing. When the top plate is in the normal horizontal position, the lower spring legs tend to act directly downwardly onto the bearing surface which maximizes the spring forces acting upwardly on the top plate. However, as the top plate and back support member pivot downwardly during rearward tilting of the chair, the rear springs also swing downwardly below the height of the control housing which thereby deflects the lower spring legs. In particular, the lower spring legs deflect from a generally horizontal orientation to a steeply inclined position such that the lower spring legs act more on a side of the arcuate bearing surface instead of the top thereof. Since a substantial portion of the force applied by the lower spring leg now acts forwardly instead of downwardly, the upward acting forces provided by the rear springs are significantly reduced so as to define a dwell for a user. Accordingly, once the chair is tilted rearwardly to its rearward position, a significant reduction in the forces applied by the rear springs occurs which makes it easier for a user to maintain the chair in the rearward position.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.